flow control problem which we can transmit over the Consider a sequenceof highlighted links pipes Intersection of links is managed by

Transcription

1 H T t s src Consider a flow control problem is described below SRC wants to send packets to DST over a sequenceof highlighted links pipes Intersection of links is managed by a router that directs flow packets to appropriate pipes clinks 1 what is the maximum rate at which we can transmit over the between SRC DST

2 1 Naive approach Transmit one feachet Whit for ACK and then transmit the rent packet Tx Pk f l Rx RTT I p sequence t n next expected plot one frucket sent in every RTT Round Trip Time Throughout 1 RTT

3 haploit pipelining we can keep multiple packets in flight on the pipe need not wait for ACK to arrive for each packet host packets can be retransmitted How many packets to keep in flight Pt called congestion iomfywwo Ideally cwms Bandwidth BW RTT Bwa RTT Throughput RTT BURTT BW Throughout approaches bandwidth when we keep optimal number of packets in flight

4 fitimating cwms is hard because Bandwidth of each link within a flow may be different BW glow min Bwana link C flow Bandwidth of births available to a flow varies with time because the links are shared by other flows RTT can vary over time effect of non optimal cwms A low value of CWND means that links are under utilized A high value of CWND results in an attempt to overflow the links

5 Since links have limited bandwidth this result in packet drops at intermediate routers Thus bandwidth of all links that carried the dropped faveket tilt the router is wasted How to estimate correct curve Three phases 1 show start start with corn D 1 For each ACK increase CWND by l This results in doubling cund fr each RTT

6 Tx Rx WND I RTT plat l cwms L i µ Hour start timing diagram 2 Additive increase when cwms reaches a certain threshold with slow start CWND moves to an additive phase where CWND inches by 1 for each RTT instead of doubling This is done in anticipation

7 of link overflows with exponential increase 3 Multiplication decrease cwms is reduced by half Cor reset to 1 under packet drops a slow start 1 additive increase starts once again packet drops are detected based on Ark timeouts on duplicate Acks generated in response to packets received out of seque cwms saw start cww wpyfdrops Evolution of an ND with time

8 Wireless links TCP The key resumption in wired network is that links are bowlers packet drops happen only if there is an overflow In contrast wireless links can be bossy In response to link losses TCP assumes its a congestion and decreases CWND However for link losses we need to do exactly opposite and increase CWND

9 SNIP Wireless link aware TCP protocol Works for networks where only last hop is wireless Eg WiFi 4G No change to be done on server or clients and they run unmo Tcp Wireless To server H q D Basestation ion Consider a downlink traffic from a server to a mobile client The basestation would monitor transmissions from server to client

10 as well as Aces from client to server Caches packets at fuse station and performs local retransmissions in the face of wireless link failures pub arrives i.ee tariff counter Yes exmi sequence Yes CacheVI Pkt 2 Forward to Mobile t.markoscong.to 12 Forward plot congestion loss co se Flow chart for swoop data

11 ACK t fiscard f Spurious ACK a No Dup Ack 1 Freebuffers updates ate 3 Propagate aeleto Sender Common case F l one EE.FI n'iith haterdnp priority actgfo lost packet Next plot lost Flowchart for SNOOP ACK